The papermaking industry employs cellulose fibres originating from wood as a principal source of raw materials. Among these, fibres originating from recycled waste paper constitute an increasingly important source. This waste paper is usually disintegrated in an aqueous phase, in most cases in a pulper, so as to obtain a pulp which is generally employed as such or as a mixture with unrecycled cellulose pulp for the manufacture of packaging paper and cardboard and of newsprint. Since the proportion of recovered cellulose fibres frequently reaches a high level which it is very difficult to exceed, other outlets have also been envisaged for waste paper, namely: its incorporation in pulps intended for the manufacture of printing and writing paper and of tissue paper. In this case it is important to have available pulp which exhibits a high brightness level. This high brightness level can usually be achieved by means of deinking and/or bleaching treatments.
Recycling of waste paper encounters a considerable obstacle in the presence, in a high proportion thereof, of dyes of various kinds. These prove very difficult to neutralise, given that they remain solidly attached to the cellulose fibres and can be stripped only under operating conditions which are very frequently damaging to the cellulose fibres themselves.
In the case of some categories of waste paper such as, for example, waste paper from carbonless copy paper (CCP), it has been possible to strip it by subjecting it to a treatment by means of a peroxide compound in an alkaline medium (U.S. Pat. 4,381,969 in the name of Interox).
A treatment capable of permitting the colour stripping of coloured waste paper from all sources consists in pulping this paper and next subjecting it to a treatment by means of an alkali metal or alkaline-earth metal hypochlorite (Pulp and Paper Manufacture, 3rd Edition, Vol. 3, Secondary Fibers and Non-Wood Pulping, F. Hamilton, B. Leopold and M.J. Kocurek, 1987, The Joint Textbook Committee of The Paper Industry TAPPI CPP.A, Atlanta and Montreal, page 234, 3rd paragraph, R.P. Singh : A. Color Stripping).
In many cases, waste paper cannot be stripped sufficiently completely for a treatment by means of a peroxide compound in an alkaline medium and it is quite frequently necessary to resort to additional treatments which are often detrimental to the cellulosic matter.
The stripping of pulps originating from waste paper by means of hypochlorite, for its part, is responsible for a considerable degradation of cellulose fibres and limits the use of paper stripped in this way to lower quality paper grades.